Asymmetric Synthesis of the Lycopodium Alkaloid, Na-Acetyl-Nb-methylphlegmarine

Decahydroquinoline Ring 13C NMR Spectroscopic Patterns for the Stereochemical Elucidation of Phlegmarine-Type Lycopodium Alkaloids: Synthesis of (-)-Serralongamine A and Structural Reassignment and Synthesis of (-)-Huperzine K and (-)-Huperzine M (Lycoposerramine Y)

Analysis of ¹³C NMR spectroscopic data of the phlegmarine subset of Lycopodium alkaloids revealed spectral patterns that allowed the stereochemical arrangement of the four stereogenic carbons in the decahydroquinoline core to be established. A relatively simple predictive set of chemical shift combinations is reported, providing a tool for the challenging stereochemical assignment of phlegmarine-type alkaloids. Based on the chemical shifts in their NMR spectroscopic profiles, the alkaloids huperzine K and huperzine M, formally reported as cis derivatives, were structurally reassigned as trans-decahydroquinolines. The NMR spectroscopic data for huperzine M were identical to those reported for lycoposerramine Y and, hence, also implied the configurational reassignment of the latter. The revised structures of the above alkaloids were confirmed by enantioselective total synthesis. Additionally, the synthesis of (-)-serralongamine A via a common intermediate precursor is reported.

Total Synthesis of (±)-Cermizine B

A practical synthesis of (±)-cermizine B was achieved. The nine-step synthesis mainly comprised two uninterrupted Michael additions including a highly diastereoselective 1,4-addition of 2-picoline to methyl 4-methyl-6-oxocyclohex-1-ene-1-carboxylate, Krapcho decarboxylation, a double reductive amination that resulted in ring closure and dearomatization of pyridine in 24% overall yield.

Access to Enantiopure 5-, 7-, and 5,7-Substituted cis-Decahydroquinolines: Enantioselective Synthesis of (-)-Cermizine B

Stereoconvergent cyclocondensation reactions of (R)- or (S)-phenylglycinol with appropriately substituted cyclohexanone-based δ-keto esters are the key steps of short synthetic routes to enantiopure 5-, 7-, and 5,7-substituted cis-decahydroquinolines. The factors governing the stereoselectivity of the cyclocondensation are discussed. The potential of the methodology is illustrated by a protecting-group-free synthesis of the phlegmarine-type Lycopodium alkaloid (-)-cermizine B.

Cyclic enaminones. Part II: applications as versatile intermediates in alkaloid synthesis

Among many other strategies, the enaminone approach is an important strategy to construct and diversify the azacyclic core in various alkaloids syntheses. In this brief review we discuss the application of cyclic enaminones as building blocks, as well as potential intermediates in the total synthesis of selected alkaloids.

Building polyfunctional piperidines: a stereoselective strategy of a three-component Mannich reaction inspired by biosynthesis and applications in the synthesis of natural alkaloids (+)-241D; (−)-241D; isosolenopsin A and (−)-epimyrtine

A stereoselective strategy for building polyfunctional piperidines by a three-component Mannich reaction and applications in the stereoselective synthesis of natural alkaloids.

Strategic innovation in the total synthesis of complex natural products using gold catalysis

This review has been organized from the perspective of synthetic target families, with emphasis on the use of gold-catalyzed transformations and cascade reactions that significantly increase molecular complexity.

A gram-scale route to phlegmarine alkaloids: rapid total synthesis of (−)-cermizine B

The asymmetric total synthesis of the cis-phlegmarine alkaloid cermizine B was achieved rapidly and on a gram-scale by an uninterrupted eight-step sequence and a final reduction step.

Tandem dinucleophilic cyclization of cyclohexane-1,3-diones with pyridinium salts

The cyclization of cyclohexane-1,3-diones with various substituted pyridinium salts afforded functionalized 8-oxa-10-aza-tricyclo[7.3.1.0(2,7)]trideca-2(7),11-dienes. The reaction proceeds by regioselective attack of the central carbon atom of the 1,3-dicarbonyl unit to 4-position of the pyridinium salt and subsequent cyclization by base-assisted proton migration and nucleophilic addition of the oxygen atom to the 2-position, as was elucidated by DFT computations. Fairly extensive screening of bases and additives revealed that the presence of potassium cations is essential for formation of the product.

Stereoselective, nitro-Mannich/lactamisation cascades for the direct synthesis of heavily decorated 5-nitropiperidin-2-ones and related heterocycles

A versatile nitro-Mannich/lactamisation cascade for the direct stereoselective synthesis of heavily decorated 5-nitropiperidin-2-ones and related heterocycles has been developed. A highly enantioenriched substituted 5-nitropiperidin-2-one was synthesised in a four component one-pot reaction combining an enantioselective organocatalytic Michael addition with the diastereoselective nitro-Mannich/lactamisation cascade. Protodenitration and chemoselective reductive manipulation of the heterocycles was used to install contiguous and fully substituted stereocentres in the synthesis of substituted piperidines.

Stereoselective synthesis of the eastern quinolizidine portion of himeradine A

The synthesis of the C(15)-C(17)/N(1')-C(11') quinolizidine portion of himeradine A is disclosed. An intramolecular, heteroatom Michael addition was employed to establish the C(6') stereogenic center with high diastereoselectivity. The quinolizidine ring was constructed using microwave-induced cyclization at the N(1')-C(2') position. The C(17) stereogenic center was introduced through a diastereoselective Overman rearrangement.

Asymmetric synthesis of all the known phlegmarine alkaloids

The asymmetric synthesis of all four of the known natural phlegmarines and one synthetic derivative has been accomplished in 19-22 steps from 4-methoxy-3-(triisopropylsilyl)pyridine. Chiral N-acylpyridinium salt chemistry was used twice to set the stereocenters at the C-9 and C-2' positions of the phlegmarine skeleton. Key reactions include the use of a mixed Grignard reagent for the second N-acylpyridinium salt addition, zinc/acetic acid reduction of a complex dihydropyridone, and a von Braun cyanogen bromide N-demethylation of a late intermediate. These syntheses confirmed the absolute stereochemistry of all of the known phlegmarines.

Synthesis of 6- and 7-membered cyclic enaminones: scope and mechanism

Six- and seven-membered cyclic enaminones can be prepared using common, environmentally benign reagents. Amino acids are used as synthetic precursors allowing diversification and the incorporation of chirality. The key reaction in this multistep process involves deprotection of Boc-amino ynones and subsequent treatment with methanolic K(2)CO(3) to induce cyclization. A β-amino elimination side reaction was identified in a few labile substrates that led to either loss of stereochemical purity or degradation. This process can be mitigated in specific cases using mild deprotection conditions. NMR and deuterium-labeling experiments provided valuable insight into the workings and limitations of this reaction. Although disguised as a 6-endo-dig cyclization, the reagents employed in the transformation play a direct role in bond-making and bond-breaking, thus changing the mode of addition to a 6-endo-trig cyclization. This method can be used to construct an array of monocyclic and bicyclic scaffolds, many of which are found in well-known natural products (e.g., indolizidine, quinolizidine, and Stemona alkaloids).

Development of an enantioselective route toward the Lycopodium alkaloids: total synthesis of lycopodine

Synthesis of a C(15)-desmethyl tricycle core of lycopodine has been accomplished. Key steps in the synthetic sequence include organocatalytic, intramolecular Michael addition of a keto sulfone and a tandem 1,3-sulfonyl shift/Mannich cyclization to construct the tricyclic core ring system. Synthetic work toward this natural product family led to the development of N-(p-dodecylphenylsulfonyl)-2-pyrrolidinecarboxamide, an organocatalyst which facilitates enantioselective, intramolecular Michael additions. A detailed mechanistic discussion is provided for both the intramolecular Michael addition and the sulfone rearrangement. Finally, the application of these discoveries to the enantioselective total synthesis of alkaloid lycopodine is described.

Diels-Alder reactions of N-acyl-2-alkyl(aryl)-5-vinyl-2,3-dihydro-4-pyridones

Readily available N-acyl-5-vinyl-2,3-dihydro-4-pyridones undergo Diels-Alder cyclization with various dienophiles to afford novel octahydroquinolines containing synthetically useful functionality. With dihydropyridone 5 and cis-disubstituted dienophiles, the resulting cycloadducts were obtained as single diastereomers in good to excellent yield. The corresponding reaction of 5 with methyl acrylate, acrylonitrile, and phenyl vinyl sulfone showed modest preference for the endo adducts. The effect of the dihydropyridone C-2 and C-4 substituents on the degree of diastereofacial control was examined. By using this methodology, the core decahydroquinoline skeleton of gephyrotoxin was prepared in a stereocontrolled fashion. Interesting reactivity was observed with certain dienophiles leading to ring-opening of the initially formed cycloadducts. This tandem reaction provides a route to uniquely substituted beta-aminoketones, alcohols, and unnatural amino acids.

The Rearrangement Route to 3-Carboxy- and 3-Hydroxymethyl-2-azabicyclo[2.1.1]hexanes: 3,5-Methanoprolines

Improved stereocontrolled syntheses of 5-anti-hydroxy-3-exo-methoxycarbonyl-2-azabicyclo[2.1.1]hexanes have been effected from pyridine. The key step in the electrophilic addition-rearrangement of 2-azabicyclo[2.2.0]hex-5-ene precursors incorporates either a 3-endo-phenyl group, as an acid precursor, or a 3-endo-phenyldimethylsilylmethyl group, as a potential hydroxymethyl and acid precursor.

Development of Highly Stereoselective Asymmetric 6π-Azaelectrocyclization of Conformationally Flexible Linear 1-Azatrienes. From Determination of Multifunctional Chiral Amines, 7-Alkyl cis-1-Amino-2-indanols, to Application as a New Synthetic Strategy: Formal Synthesis of 20-Epiuleine

The highly stereoselective asymmetric 6pi-azaelectrocyclization was achieved as a general synthetic method based on the reaction between the (E)-3-carbonyl-2,4,6-trienal compounds and the (-)-7-alkyl-cis-1-amino-2-indanol derivatives which are effective chiral amines. The 7-alkyl-substituted 2-indanol moiety of the cyclized products was efficiently removed by the novel manganese dioxide oxidation under remarkably mild conditions, and the method was successfully applied to the formal synthesis of optically active 20-epiuleine.

Ten new Lycopodium alkaloids having the lycopodane skeleton isolated from Lycopodium serratum Thunb

Ten new alkaloids, lycoposerramines-F (1), -G (2), -H (3), -I (4), -J (5), -K (6), -L (7), -M (8), -N (9), and -O (10), having lycopodine-related structures, were isolated from the club moss Lycopodium serratum THUNB. and their structures were elucidated on the basis of spectroscopic analysis and/or chemical transformation.

Sieboldine A, a novel tetracyclic alkaloid from Lycopodium sieboldii, inhibiting acetylcholinesterase

[structure: see text] A novel Lycopodium alkaloid with an unprecedented fused-tetracyclic ring system consisting of an aza-cyclononane ring having a N-hydroxy group, a cyclohexanone, a cyclopentanone, and a tetrahydrofuran ring, sieboldine A (1), was isolated from the club moss Lycopodium sieboldii. The structure and relative stereochemistry were elucidated on the basis of 2D NMR data and X-ray analysis. Sieboldine A (1) exhibited a potent inhibitory activity against acetylcholinesterase and modest cytotoxicity.

Diels-Alder reactions of 5-vinyl-1-acyl-2-aryl-2,3-dihydro-4-pyridones: regio- and stereoselective synthesis of octahydroquinolines

[reaction: see text] Readily available 5-vinyldihydropyridones 2 undergo Diels-Alder cyclization with various dienophiles to afford novel octahdroquinolines. The process is highly stereoselective and provides heterocyclic products containing synthetically useful functionality.

Significant acceleration of 6 pi-azaelectrocyclization resulting from a remarkable substituent effect and formal synthesis of the ocular age pigment A2-E by a new method for substituted pyridine synthesis

The remarkable acceleration of 6 pi-azaelectrocyclization due to the combination of the C4-carbonyl and the C6-alkenyl or phenyl substituents in 1-azatrienes was found. This observation was rationalized by considering the remarkable orbital interaction between the HOMO and LUMO of 1-azatrienes, which were obtained by molecular orbital calculations. The formal synthesis of the unusual retinal metabolite, A2-E, was achieved by two types of the new one-pot synthesis of substituted pyridines by utilizing the obtained facile 6 pi-azaelectrocyclization, one of which is compatible with the proposed metabolic pathway of A2-E.

Total syntheses of the Securinega alkaloids (+)-14,15-dihydronorsecurinine, (-)-norsecurinine, and phyllanthine

A new strategy for enantiospecific construction of the Securinega alkaloids has been developed and applied in total syntheses of (+)-14,15-dihydronorsecurinine (8), (-)-norsecurinine (6), and phyllanthine (2). The B-ring and C7 absolute stereochemistry of these biologically active alkaloids originated from trans-4-hydroxy-L-proline (10), which was converted to ketonitrile 13 via a high-yielding eight-step sequence. Treatment of this ketonitrile with SmI2 afforded the 6-azabicyclo[3.2.1]octane B/C-ring system 14, which is a key advanced intermediate for all three synthetic targets. Annulation of the A-ring of (-)-norsecurinine (6) with the required C2 configuration via an N-acyliminium ion alkylation was accomplished using radical-based amide oxidation methodology developed in these laboratories as a key step, providing tricycle 33. Annulation of the D-ring onto alpha-hydroxyketone 33 with the Bestmann ylide 45 at 12 kbar gave (+)-14,15-dihydronorsecurinine (8). In the securinine series, the D-ring was incorporated using an intramolecular Wadsworth-Horner-Emmons olefination of phenylselenylated alpha-hydroxyketone 47. The C14,15 unsaturation was installed late in the synthesis by an oxidative elimination of the selenoxide derived from tetracyclic butenolide 50 to give (-)-norsecurinine (6). The A-ring of phyllanthine (2) was formed from hydroxyketone 14 using a stereoselective Yb(OTf)3-promoted hetero Diels-Alder reaction of the derived imine 34 with Danishefsky's diene, affording adduct 35. Conjugate reduction and stereoselective equatorial ketone reduction of vinylogous amide 35 provided tricyclic intermediate 36, which could then be elaborated in a few steps to stable hydroxyenone 53 via alpha-selenophenylenone intermediate 52. The D-ring was then constructed, again using an intramolecular Wadsworth-Horner-Emmons olefination reaction to give phyllanthine (2).